The present invention relates to an implantable cochlear stimulator (ICS) and a method for fitting such ICS to a particular patient, where "fitting" refers to the process of determining and setting the amplitude or intensity of the stimuli generated by the ICS to a level or setting that is both effective (allows the ICS to optimally perform its intended function) and comfortable (not excessively loud or painful) for the patient. More particularly, the invention relates to a method of self-fitting an ICS to a particular patient using objective feedback rather than subjective feedback in order to determine stimulation parameters for the patient. The invention further relates to an ICS that includes implantable self-fitting circuitry.
An ICS is an electronic device that helps a profoundly deaf patient to achieve the sensation of hearing by applying electrical stimulation directly to the auditory nerve through the cochlea. An ICS includes electronic circuitry, hermetically sealed for implantation, and an electrode array (comprising a plurality of spaced-apart, independent, individual electrodes) suitable for insertion into the cochlea. An ICS system includes a microphone (for sensing audio sounds), a speech processor (for processing the sensed audio sounds and converting such to electrical stimulation signals), and a cochlear stimulator (for receiving the electrical stimulation signals and directing them to the appropriate electrode or electrodes of the electrode array). Typically, the microphone and speech processor are external components worn or carried by the patient, and the electrical stimulation signals produced by the speech processor are coupled into the implanted cochlear stimulator through an inductive, rf, or other wireless link.
Cochlear stimulators are known in the art, as evidenced, e.g., by U.S. Pat. Nos. 3,751,605 (Michelson); 4,400,590 (Michelson); 4,267,410 (Forster et al.); 4,284,856 (Hochmair et al.); 4,408,608 (Daly et al.); 4,428,377 (Zollner et al.); and 4,532,930 (Crosby et al.). All such stimulators generate electrical stimulation pulses that may be selectively applied to the cochlea of a patient through an appropriate electrode or electrode array.
When the implanted cochlear stimulator (ICS) is initially implanted in the patient, and during follow-up tests and checkups thereafter, it is usually necessary to fit the ICS to the patient. Such "fitting" includes adjustment of the base amplitude or intensity of the various stimuli generated by the ICS from the factory settings (or default values) to values that are most effective and comfortable for the patient. For example, the intensity or amplitude and/or duration of the individual stimulation pulses provided by the ICS must be mapped to an appropriate dynamic audio range so that the appropriate "loudness" of sensed audio signals is perceived. That is, loud sounds should be sensed by the patient at a level that is perceived as loud, but not painfully loud. Soft sounds should similarly be sensed by the patient at a level that is soft, but not so soft that the sounds are not perceived at all.
Fitting and adjusting the intensity of the stimuli and other parameters of a cochlear implant to meet a given patient's needs thus requires determining the electrical stimulation levels at which "sound" is perceived (threshold), at which a comfortable sound level (comfort level) is perceived, and the perceptual loudness growth function resolution within the patient's dynamic range. Heretofore, these psychophysical parameters have been determined by an expert clinician presenting various stimuli to the patient and relying on subjective feedback from the patient as to how such stimuli are perceived. Such subjective feedback typically takes the form of either verbal (adult) or non-verbal (child) feedback. Unfortunately, relying on subjective feedback in this manner is difficult, particularly for those patients who may have never heard sound before and/or who have never heard electrically-generated "sound". For young children, the problem is exacerbated by a short attention span, as well as difficulty in understanding instructions and concepts, such as high and low pitch, softer and louder, same and different. Furthermore, in the developing nervous system of young children, frequent changes in the intensity of the stimuli may be required for optimal benefit. These changes may require frequent refitting sessions or, ideally, continuous adjustment during use in response to the loudness perceived by the brain.
In view of the above, it is evident that a more objective approach is needed for fitting an ICS to a patient. The present invention relates to the use of physiological signals generated by the nervous system to control the level of stimulation that the ICS applies to the cochlea. In order to better understand this concept, it will be helpful to review these phenomena. When neurons are activated by natural or artificial means, they generate pulses of electrical current called action potentials. The current produced by a single neuron is very small, but electrical stimulation, such as is applied by an ICS, tends to recruit large numbers of neurons synchronously. This results in a compound action potential (CAP) that can be recorded electronically in the tissues surrounding the neurons, particularly in the fluid-filled cochlear ducts where the stimulating electrodes of an ICS are usually located. The amplitude of this compound action potential, or CAP, is approximately related to the number of auditory neurons that have been activated by the electrical stimulation. The level of stimulation at which a CAP can first be recorded corresponds approximately to the threshold for hearing (T), or a small, fixed value above that level.
The action potentials produced by auditory neurons are conducted to various relay nuclei of the brainstem, which transform the information into action potentials that are transmitted by other neurons to yet further nuclei and eventually to the perceptual centers in the cerebral cortex. The compound action potentials resulting from patterns of neural activity in these subsequent nuclei can also be recorded electronically, but they are very much weaker, less accessible and more variable. Typically, these are recorded by widely spaced external electrodes on the scalp and enhanced by stimulus-triggered averaging, in which the small and noisy signals recorded following each of thousands of identical stimuli are added together in order to reduce the effects of noise inherent in the electrodes and amplifiers used to detect the scalp potentials. The amplitude of these electrical auditory brainstem responses (EABRs) depends not only on the number of auditory neurons that are initially stimulated, but also on the size and condition of the nuclei, the connections between them, and on descending signals from the perceptual centers that can influence the transformations produced in the relay nuclei. Obtaining and using EABRs to fit an ICS system is tedious and controversial, particularly in children with uncertain developmental status of the brainstem nuclei.
When the nerve signals finally arrive in the perceptual centers, they give rise to the conscious perception of sound and its apparent loudness. When sounds are perceived as being undesirably loud, the brain can employ various mechanisms to reduce the intensity to more desirable levels. In normal hearing, the perceived loudness of sound depends on the amount of acoustic energy that is transmitted through the middle ear to the cochlea. The brain can control this via the mechanical tension produced by two muscles in the middle ear: (1) the stapedius, and (2) the tensor tympani. The brain sends neural signals to these muscle fibers, causing them to produce active mechanical tension which damps the mechanical linkage and reduces the transmission of sound energy. Even when the sensations of sound are produced electronically by an ICS, these middle-ear reflexes (MER) are usually present even though they have no effect on the electrical stimulation that is actually stimulating the auditory neurons. The level of stimulation at which the middle ear reflex, or MER, appears is associated approximately with the most comfortable loudness (MCL) level of sound perception. This reflex, in turn, may be measured by any of three different methods or means.
First, the contraction of the muscle has been observed visually when cochlear stimulation is applied during the surgical implantation of the ICS. This is problematic, however, because it depends on the level and nature of the anesthesia.
Second, the contraction of the muscle has been inferred from measurements of the acoustic impedance during cochlear stimulation. This requires attaching a tube to the external ear canal to apply air pressure and measure small changes in the response to pressure impulses. Children must usually be sedated, which may interfere with the reflex, and the recorded response depends on the mechanical details of the middle ear linkage, which may be poorly developed or damaged as a result of the deafness.
Third, the electrical activity that accompanies the muscle contractions can be recorded as the electromyogram (EMG).
The first two methods described above have been employed in the fitting of ICS systems, but they are not suitable for frequent recalibration. The last method has been employed by researchers studying the nervous system, but not as a clinical technique because of the relative inaccessibility of the middle ear muscles.